Require adding entries to the Kind enum to be done at the end of the enum

[abitofevrything]

Since the types of elements are returned as indices into the Kind enum in the generated index.json file, any additions to the Kind enum are effectively a breaking change for any tooling consuming that file as their indices will no longer line up with dartdoc's.
We observed this as our version of dartdoc had the new Kind.extensionType whereas the version of dartdoc used to generate the file we were consuming did not.

This can be mitigated by only ever adding new types at the end of the Kind enum, so the indices of existing element types remain the same. It'd be nice if this could be required for future additions to the enum.